Fibonacci numbers

fibonacci.js

// Fibonacci numbers

/* extended version */
function f(n) {
  if (n === 0) {
    return 0;
  } else if (n === 1) {
    return 1;
  } else {
    return f(n - 1) + f(n - 2);
  }
};

/* short version */
function f(n) {
  if (n === 0) return 0;
  return (n === 1) ? 1 : f(n - 1) + f(n - 2);
}

const fn = f(11);
console.log(fn); // 89

/* Promise-based version */

function getFibonacciNumber(n) {
  return Promise.resolve().then(() => {
    return f(n);
  });
}

getFibonacciNumber(11).then((result) => {
  console.log(`Result: ${result}`); // "Result: 89"
});